Conventional cutter bits comprise a generally tubular cutter bit body provided at the bottom end thereof with a plurality of cutters for cutting the well formation. Normally, the cutter bit is forged and provided at its bottom end with three legs having attached to their bottom portions, known as shirt tails, mounting pins for securing cone cutter elements made of a durable material. These cutter elements may be conical and are provided with hard cutting teeth, made, for example, from tungsten carbide, for cutting a formation. Through the centre of the cutter bit body is usually provided a bore through which drilling fluids are delivered at high velocity via three conduits to areas adjacent to the cutters to assist in cutting and cuttings removal. However, the central bore is not itself directly open to the cutting face and, for this reason, a number of problems can arise when a cutter bit is being run into the hole, pulled from the hole or simply being used in cutting operations.
Firstly, it is to be appreciated that the cutter bit body will be in gauge with the walls of the hole and, therefore, as the cutter bit is run into the hole it acts as a piston which compresses material below the bottom end of the cutter bit body. The only relief from the increasing pressures is leakage through zones where the cutter bit body has lesser diameter than the gauge of the hole. However, as these zones only have very small area for flow of cuttings and fluids, the flow is substantially restricted and little pressure relief is obtained. The consequence of such increasing pressures is "surge" or the generation of stresses on the rock formations along the open hole section below the cutter bit. It is known in the oil and gas industry that these stresses or surge pressures cause major damage to weak formations and, more importantly, to productive reservoir sands. In particular, the formation may be damaged by fracturing of the formation beneath the cutter bit during running into the hole. This problem is worsened by increasing running speeds and higher viscosity drilling fluids that cause higher pressure drop across the cutter bit. Clearly, damage to the formation, with consequential reductions in oil and gas recovery, is unacceptable to the industry for cost reasons.
Secondly, upon pulling of the cutter bit from the hole a reverse problem known to the industry as "swabbing" occurs. In the worst cases, swabbing, which results from fluid flow into the reduced pressure area caused by pulling of the cutter bit and drillstring from the hole can cause blowouts which are extremely hazardous. In addition, swabbing may result in contamination of the drilling fluids by formation fluids necessitating costly treatment processes and/or increased drilling fluid cost. Swabbing is most pronounced when the cutter bit or drilling stabilisers are encrusted or packed with formation cuttings. In these cases, it becomes increasingly more difficult to maintain an even hydrostatic pressure on both sides of the cutter bit because the already restricted fluid flow area past the outer circumference of the cutter bit is restricted even further. Other factors that contribute to swabbing include variable viscosity drilling fluids and variation in hole diameter.
A yet further problem with conventional cutters is poor hole cleaning due to a poor efficiency of cuttings removal from the centre of the cutting area at the bottom of the hole. Conventional cutter bits rely on the delivery of drilling fluid to the bottom of the hole for effective hole cleaning. In a typical design, as discussed above, there are three cone cutters and, therefore, three nozzles, each adjacent to each cutter cone, are provided for the jetting of cooling and cleaning fluid to the bottom of the hole. Jetting occurs at high velocity with fluid impacting the bottom of the hole at or near its outer edges close to the hole walls. This action effectively washes the outer zone of the hole to remove cuttings. However, cleaning from the centre of the hole is much less efficient because cuttings become trapped in recesses between the cutters and the bottom end of the cutter bit body. In conventional bits, cutting accumulation in this area can create what is called in the industry bit bailing, where the trapped cuttings restrict the rotation of the cutting cones. Bit bailing has been a major problem in the industry. Thus the overall cleaning and cutting efficiency of the cutter bit is reduced.